Here's some news we're tracking today. NATO and Ukrainian officials are warning about a sizable troop build-up by Russia along its border with Ukraine. Western estimates put the military presence on the Russian side at between 20 and 50 thousand troops. Sources told Reuters these include infantry and armored units along with some air support.

Now, why the Russian forces would have gathered is still not clear. Although some Western officials fear they're preparing to invade Ukraine's Russian-speaking east.

LINDA WERTHEIMER, HOST:

We've heard a lot from our correspondents inside Ukraine, and this morning we got NPR's Soraya Sarhaddi Nelson on the line from the other side of the border. She is inside Russia in the western Russian city of Belgorod near where troops were sighted in recent weeks. Good morning, Soraya.

SORAYA SARHADDI NELSON, BYLINE: Good morning, Linda.

WERTHEIMER: Now, you've been traveling around the area where the troops are massed across the border. Over the last 24 hours, have you seen them?

NELSON: Not a single one. And the only military vehicles that I've seen are historic tanks in front of a World War II museum. But it's really not a surprise because Russian officials are denying any build-up and are certainly not doing it publically. Plus, reporters are being refused permission to travel any closer than three miles to the border, which is likely where a lot of these military units are.

WERTHEIMER: Do you get the impression that local folks are aware of them, have seen them?

NELSON: Some have seen armored vehicles and other military equipment being transported by train and they've also noticed an absence of soldiers in recent weeks at like the local military post here. But most of the people say they've learned about the build-up from the Internet. In any event, few want to comment about it because they say it's an incredibly sensitive topic here.

And just how sensitive became clear when I stopped in a neighborhood about 10 miles from the border, where a train was seen carrying armored vehicles in the direction of Ukraine earlier this month.

(SOUNDBITE OF MEN SPEAKING FOREIGN LANGUAGE)

NELSON: Our local producer and I had barely started interviewing a resident when six stern-looking Russian policemen, some of them were wearing camouflage, pulled up at high speeds in an SUV and a van and then came over to us.

WERTHEIMER: What on earth happened?

NELSON: Well, they held us at that location for about three hours and they were going over our passports and Russian paperwork and IDs and asking us over and over again what we were doing there, why we were asking questions and what kind of questions we were asking. The first six officers who showed up worked for the transportation police and criminal investigation division and they were later joined by other police units, including plain clothed federal security agents.

In the end, they did let us go and we just continued to do our interviews for a few hours.

WERTHEIMER: Soraya, what about the Russians? How are they reacting to the troops' presence?

NELSON: Well, most of the ones that I've interviewed here say they're comforted that there are troops around because it offers protection they feel they need from quote-unquote unsavory elements in Ukraine. Nobody I spoke to favors an incursion into Ukraine, but they said that if Eastern Ukrainians vote to follow the path of Crimea and join the Russian federation, they would welcome them here.

I interviewed one man next to the railway overpass where Russian armored vehicles were taken by train in the direction of the border. I can't identify him because he fears police retribution.

UNIDENTIFIED MAN: (Speaking foreign language)

NELSON: He says people were shocked when they saw the military vehicles and equipment passing by here. He describes them as belonging to elite Russian paratroopers. The Russian man adds there aren't normally military units in this area so the appearance of these vehicles and equipment raises the anxiety people are feeling over what many think or fear is a fascist takeover in neighboring Ukraine.

WERTHEIMER: That's NPR's Soraya Sarhaddi Nelson speaking to us from the Russian city of Belgorod. Soraya, thank you very much.